The present invention relates to a spray device for coating articles with powder and, more particularly, to a spray device in which powder particles are electrokinetically charged as they are moved along a feed channel.
A spray device utilizing electrokinetic charging of powder particles is known from U.S. Pat. No. 4,090,666, which discloses a length of pipe forming a double annular-slot nozzle through whose inner channel the vehicle-gas powder stream, and through whose outer annular-slot the stream of accelerating gas, are fed. A powder spray device with electrokinetic charging of the powder is also shown in Federal Republic of Germany Pat. No. 2,347,491. Furthermore, Federal Republic of Germany No. OS 1,814,809 shows a spray device in which supplementary air is added into the stream of powder and thereupon the stream of powder and supplementary air is converted by guide vanes into an eddy flow.
The object of the present invention is to obtain a stronger electric charging of the particles of powder over a shorter path of the feed channel.
This object is achieved in accordance with the invention in a spray device in which a powder passage cross section of a feed channel changes along the axial length of the feed channel, due to an elongated guide body arranged axially at its center between injector and discharge opening, from a full cross-sectional shape to a cross-sectional shape of annular cross section with a smaller passage cross section of flow. The guide body consists of a material which electrically charges the powder by friction, and extends from close to the injector at least so far towards the discharge opening that the powder is more strongly charged electrically by friction on the guide body and the powder loses substantially no electric charge from the downstream end of the guide body up to the discharge opening.
By the invention the following effect is obtained: The stream of gas of the injector draws the vehicle-gas powder stream in the direction towards the discharge opening and accelerates it in this direction to a higher speed. The cross section of passage of the feed channel is so narrowed by the guide body that further acceleration of the gas-powder stream takes place. The higher velocity results in a stronger electrical charging of the powder particles by friction on the channel wall and on the outer surface of the guide body. At the same time, as a result of the reduction in cross section due to the guide body a stronger concentrating of the powder particles takes place, as a result of which the powder particles slide with greater force along the powder-channel wall and the surface of the guide body and are more strongly electrically charged by friction. Furthermore, the electrical charging by friction takes place not only on the inner surface of the wall of the feed channel but also on the not much smaller surface of the guide body so that there is obtained practically a doubling of the frictional surface and thus also a substantial increase in the electrical frictional charging. By the conveying action of the acceleration gas and the damming effect of the guide body, less vehicle gas is required for the conveying of the powder, and thus also less energy. Furthermore, the smaller amount of vehicle gas has a favorable effect both on the aforesaid electrical frictional charging and on the cloud of powder emerging from the discharge opening of the device or the emerging powder spray jet and on the quality of the surface coated with the powder since fewer powder particles bounce off from the article due to the smaller amount of gas.
Other features are set forth in the subordinate claims.